1. Field of the Invention
The present invention relates to an exhaust turbocharger and a turbocharging system.
2. Description of the Related Art
In known exhaust turbines for internal combustion engines, turbine shafts are rotatably supported by radial bearings, and the radial bearings are lubricated with oil, in which there is a problem in that white smoke is produced when the lubrication oil leaks from turbine-impeller sides into exhaust pipes. Accordingly, a known exhaust turbine for an internal combustion engine is disclosed in, for example, JP-A-48-72511, in which the lubrication oil is prevented from leaking to the turbine impeller side by making the lubrication oil spattering toward a radial bearing side by using a groove formed in a stepped portion disposed on a turbine shaft.
However, in the exhaust turbine disclosed in, for example, JP-A-48-72511, there is a risk in that the spattering lubrication oil penetrates into a gap between an outer surface of the turbine shaft and a bearing housing. The problem is caused by a fact in that although a seal ring is provided between the turbine shaft and the bearing housing, a pressure difference xcex94P (Ptxe2x88x92Ph) at the seal ring changes in pulsation between positive and negative pressures, the pressure difference xcex94P being between pressure Pt at the turbine impeller side of the seal ring which changes due to exhaust-pulsation and pressure Ph at an oil drain side of the seal ring which is substantially constant at ambient pressure. The lubrication oil leaks to the turbine impeller side when the pressure difference AP becomes negative.
The inventors have confirmed that the oil is more likely to penetrate into the gap between the outer surface of the turbine shaft and the bearing housing, as the rotation of turbine is more reduced. Recently, in motor vehicles, engine-idling speed tends to be reduced in order to reduce fuel consumption. Consequently, the rotational speed of turbines decreases, whereby lubrication oil is likely to leak.
Accordingly, it is an object of the present invention to provide an exhaust turbocharger for an internal combustion engine, in which leakage of lubrication oil can be suppressed.
To the end, according to an aspect of the present invention, an exhaust turbocharger for an internal combustion engine comprises a turbine shaft fixed to a turbine impeller to be driven for rotation by exhaust gas of the internal combustion engine; a radial bearing mounted to a bearing housing, for supporting the turbine shaft in radial directions, the bearing housing including an oil drain for discharging oil which has lubricated the radial bearing; and a stepped part formed on the turbine shaft between the turbine impeller and the radial bearing so that the outer diameter of the turbine shaft is greater at the turbine impeller side than at the radial bearing side. The distance L from an end of the radial bearing to the stepped part is set to a value at which oil moving from the end of the radial bearing does not reach the stepped part when a turbine-revolution speed Nt is higher than that Nti which is produced by an idling operation of the internal combustion engine.
With this arrangement, oil-spatter can be suppressed, whereby oil leakage can be avoided.
According to another aspect of the present invention, an exhaust turbocharger for an internal combustion engine comprises a turbine shaft fixed to a turbine impeller to be driven for rotation by exhaust gas of the internal combustion engine; a radial bearing mounted to a bearing housing, for supporting the turbine shaft in radial directions, the bearing housing including an oil drain for discharging oil which has lubricated the radial bearing; and a stepped part formed on the turbine shaft between the turbine impeller and the radial bearing so that the outer diameter of the turbine shaft is greater at the turbine impeller side than at the radial bearing side. The oil drain is formed so as to open toward the turbine impeller side from a supporting part of the radial bearing and to enclose the stepped part of the turbine shaft. The distance L from an end of the radial bearing toward the turbine impeller, at which the turbine shaft becomes free from the radial bearing, to the stepped part of the turbine shaft is set to a value greater than a gap produced by a difference between an inner diameter D of a hole for receiving the radial bearing and an outer diameter d of the turbine shaft when the turbine shaft is disposed coaxially with the hole.
With this arrangement, oil-spatter can be suppressed, whereby oil leakage can be avoided.
In the exhaust turbocharger for an internal combustion engine, according to the present invention, an annular plate inserted at an outer side of said turbine shaft may be provided between the end of the radial bearing and said radial bearing.
With this arrangement, oil-spatter can be suppressed, whereby oil leakage can be avoided.
According to still another aspect of the present invention, turbocharging system comprises an exhaust turbocharger for an internal combustion engine, which includes a turbine shaft fixed to a turbine impeller to be driven for rotation by exhaust gas of the internal combustion engine, and a radial bearing mounted to a bearing housing, for supporting the turbine shaft in radial directions. The turbocharging system also includes a control member for increasing idling speed after an idling operation continues for a predetermined time.
With this arrangement, oil-spatter can be suppressed, whereby oil leakage can be avoided.